Brief noxious heat pulses (2.0 sec duration; peak temperature equals 51.5 degrees C) programmed by a computer and generated by a contact thermode have been used to study peripheral and central nervous system mechanisms of first and second pain. Previous research has demonstrated that the heat-induced first and second pains are related to impulse conduction in A delta heat nociceptive afferents and C polymodal afferents respectively. When 4 identical heat pulses are applied to the same location on the hand, first pain decreases in perceived intensity while second pain summates. Previous work in our laboratory has shown that the latter is related to CNS mechanisms in the spinal cord dorsal horn. Work in progress demonstrates that both lamina I spinothalamic neurons and lamina II-III substantia gelatinosa neurons of the dorsal horn respond to heat in a manner consistent with psychophysical responses, that is, with a brief latency (A delta heat nociceptor mediated) response that shows suppression and a long latency response (C polymodal nociceptor mediated) that shows summation. Thus we have determined the neurons of origin of this summation phenomenon. Data are analyzed with the aid of a computer system.